1. Field of the Invention
The invention relates to methods and apparatii for optoelectronic digital memories (combining optic and electric ports). More particularly, compositions and fabrication methods using photoconductive thin films for high-density, high-speed storage of digital data are included within the claimed invention.
2. Photosensitive Memories
Photosensitive organic compounds in multilayer configurations have been proposed and used for information storage,.sup.7 especially for images. Photovoltaic behavior of single-layer organic films (liquid crystal porphyrins in particular) has also been described recently..sup.4 Storage of binary data as trapped charge is taught in a recent U.S. Patent for silicon nitride layers on silicon substrates,.sup.11 and information storage in materials with persistent internal polarization (PIP).sup.14 is described in references dealing with photocopying.sup.8 and computers..sup.9 Photoelectrets (photoconductors with capacity for PIP in the dark) are taught as memory elements in several U.S. Patents..sup.10, 12, 13 However, recent technical references.sup.15, 16 reveal no use of the thin film memory elements of the present invention in high-density, high-speed, optoelectronic digital memories with a capability for long-term information storage.
On the contrary, instead of being in the form of thin film memory elements, photosensitive memory media in prior art devices are generally so thick (80-650 .mu.m).sup.14 that light penetrates less than 1% of the thickness. Consequently, prior art devices use light to aid in manipulation of surface electrostatic charge, but specific charge distributions can only be maintained for a few hours in the dark. In fact, charge distributions established under photoexcitation can be altered long after the light has been extinguished (as much as a day later)..sup.8 Thus, long-term information storage with such films is not possible.
Additionally, prior art memory media are multi-component systems comprising photoconductive particles dispersed in insulating binder matrices, the matrix in each case being non-photoconductive..sup.8, 9, 10, 13, 14, 18, 19, 20 Because of the substantial memory film thickness, prior art devices require relatively high voltage for their operation (ca 300 V) and have relatively slow response times (from about a ms to a fraction of a .mu.s). A voltage applied across a thick film of prior art memory medium creates an electric dipole at each electrode contact; the two dipoles can then be detected and separately discharged..sup.8 The stored charge in prior art memory media is surface electrostatic charge, so that the surface charge density is not a function of film thickness ad is too small (10.sup.-18 to 10.sup.-18 coulomb/.mu.m.sup.2) to be used in high density memories..sup.14,20.